Despite considerable effort, there is still no reliable way to either prevent or rescue dopamine (DA) neurons from the progressive degeneration that occurs during aging or in Parkinson's Disease (PD). Since clinical diagnosis almost always occurs after the vast majority of DA neurons have been destroyed, researchers continue to work to develop ways in which to replace lost tissue with transplanted cells capable of dopaminergic function. Our goal is l to study purified populations of engineered stem, progenitor or fetal DA neurons after transplantation into the Parkinsonian rat. A major limitation of these approaches is the inability to monitor the progression, if any, of the grafted cells without highly invasive tissue biopsy, which invariably results in the death of the animal. The vast numbers of animals required for these studies could be reduced significantly (with the concomitant reduction in costs) if each animal could be studied non-invasively and repeatedly. In addition, measurements made of the DA neuronal regeneration in a rat model ex vivo are not translatable to humans. Molecular imaging, using PET and SPECT, of animal models of PD, and other neurodegenerative diseases, enables the study of the in vivo neurochemical basis of the disorder. In this proposal we aim to perform quantitative imaging of dopaminergic neurons in vivo in longitudinal studies of the same animals over an extended period of time after stem cell implantation. We aim to validate quantitative models of dopaminergic function in rats, using ultra-high resolution] PET and SPECT. [18F]DOPA imaging will be used with PET to monitor striatal dopa decarboxylase activity, while [99mTc]TRODAT-1 and SPECT will be used to measure dopamine transporter (DAT) availability directly. These will be validated against established post mortem methods, such as GFP reporter gene expression and immunocytochemistry. Longitudinal imaging studies of rats, following stem cell implantation, will enable the visualization of the regeneration of DA neurons over an extended period of time. Once the imaging techniques have been fully validated, they will be applied to a variety of stem cell implant models, and correlated with behavioral studies. This non-invasive approach will enable the best combination of cell types and growth factors to be established without sacrificing the animals. The ultimate goal of this study is to develop methods which will enable the monitoring in vivo of DA neuron replacement treatments in Parkinson's and other neurodegenerative diseases. This will provide vital information in the animal model of PD, allow us to longitudinally follow DA neuron regeneration, and, most importantly, will be translatable to clinical human studies using similar techniques.